Previously known apparatus and methods provide phase-change ink to a multiple-orifice ink-jet print head, apply heat to melt the ink in a controlled manner, and selectively jet the melted ink toward a print medium to form a printed image. Phase-change ink is particularly advantageous because of its convenience, image quality, economy, and use of conventional print media.
In particular, U.S. Pat. No. 4,418,355 for an INK JET APPARATUS WITH PRELOADED DIAPHRAGM AND METHOD OF MAKING SAME describes a multiple-orifice ink-jet print head having an elongated serpentine-shaped heater element pressed against a heat-spreading ink reservoir wall plate for melting phase-change ink contained in the reservoir. A thermistor is inserted into a centrally located well in the ink reservoir wall plate to sense the ink reservoir temperature. The ink-jet print head reciprocates back and forth across a print medium while selectively jetting ink from piezoelectric transducer-driven jets to print an image.
Skilled workers know that an ink-jet head ejects ink drops at a velocity that is determined by various parameters including the energy imparted to the ink by the piezoelectric transducer, the geometry of features in the head, and the ink viscosity. In particular, the viscosity of phase-change ink varies widely with temperature, a typical ink being solid at room temperature, rubbery near its 86 degree Celsius melting point, and a flowing liquid at its jetting temperature of about 130 degrees to about 140 degrees Celsius. Given a typical ink-jet head and a fixed amount of transducer energy, ink drop ejection velocity changes about two to about three percent per degree Celsius.
Because the ink-jet print head moves relative to the print medium while ejecting drops of ink, the landing points of the drops will vary in proportion to changes in drop ejection velocity. Therefore, to ensure acceptable drop landing accuracy, the phase-change ink temperature should be regulated and should be substantially the same for each jet of the multiple-orifice ink-jet print head. Ink temperature variations of greater than about three degrees Celsius can cause visible ink drop landing errors.
Factors causing temperature nonuniformity from jet to jet include nonuniform heat conduction losses, convection losses into the air, and radiation losses from the print head into adjacent objects. Convection losses are especially nonuniform in printers using a print head that reciprocates back and forth, thereby "fanning" the leading and trailing edges of the print head more than its central portions.
Maintaining substantially the same ink temperature for each ink jet becomes more difficult as the print head become wider to accommodate additional ink-jet orifices. U.S. Pat. No. 5,087,930 for a DROP-ON-DEMAND INK JET PRINT HEAD, which is assigned to the assignee of this application, describes a 95-millimeter wide, 96-orifice print head designed for ejecting phase-change inks. The ink-jet print head is attached to an ink reservoir which is mounted on a reciprocating carriage as described in U.S. Pat. No. 5,083,143 for ROTATIONAL ADJUSTMENT OF AN INK JET HEAD, which is assigned to the assignee of this application.
Differentially heating the 96-orifice print head to achieve a uniform ink temperature throughout the print head can be accomplished by multiple heaters, each controlled in response to a temperature sensor located adjacent to the particular heater. However, such an approach is unnecessarily complex and expensive.
Referring to FIG. 1, a prior art heater 10 was developed that generates more heat at its edges near its shorter side margins than at its central portion. A single heater foil 12 compensates for nonuniform convection losses near the shorter side margins of the 96-orifice print head and is regulated by a temperature controller employing a single sensor. Heater 10 is a conventional flex circuit in which heater foil 12 is formed from etched Inconel.RTM. (alloy 600) foil material laminated between a pair of Kapton.RTM. insulating layers. A heat-spreading copper foil layer is bonded to one of the Kapton.RTM. layers. Heater 10 is sized to match a major surface of the 96-orifice print head.
Heater foil 12 is electrically connected by a pair of contacts 14 to a temperature controller 16. Temperature controller 16 applies a pulse-duration modulated voltage across contacts 14 in response to the temperature sensed by a thermistor 18. Heater foil 12 has a set of eleven adjacent heating zones 20 (shown generally as regions bounded by dashed lines) spaced across the X-dimension (width) of heater 10. Because electrical current flow is equal everywhere along heater foil 12, the watt-density in any zone 20 is proportional to the electrical resistance of heater foil 12 in that zone. The resistance of heater foil 12 is, therefore, made larger in heater zones 20 near contacts 14 than in heater zones 20 near thermistor 18. The watt-densities of heater zones 20 vary from about 2 to 2.5 watts per square centimeter near the center of heater 10 to about 3 to 3.25 watts per square centimeter at its left and right edges.
Thermistor 18 is embedded in a well in the 96-orifice print head. Access to thermistor 18 is gained through a cutout region 22 in heater 10. The location of thermistor 18 is not critical outside of the intended control area because the temperature sensed anywhere along the width of the 96-orifice print head is equalized elsewhere along the width of print head by the zoned watt-density of heater 10. Because the phase-change ink is in intimate contact with the print head, equalizing the print head temperature also equalizes the ink temperature.
FIG. 2 shows a temperature contour profile across an orifice surface 24 of the 96-orifice print head heated by heater 10 as determined by infrared scanning measurements. Two-degree Celsius contour lines 26 show that the temperature across orifice surface 24, varies by about four degrees Celsius from a centrally located hot area 27 to edge margins 28 of the print head. Note that the orifices span an oblique region of orifice surface 24 that has a greater than four degree Celsius temperature variation. A further improvement in temperature uniformity would improve the consistency of the ink drop time to paper and, therefore, the drop landing accuracy for the 96-orifice print head.
It is also known that certain phase-change inks decompose if kept at an elevated temperature for extended periods of time. For this reason, predetermined amounts of phase-change ink are melted and stored in a reservoir at a temperature slightly above the ink melting temperature, but significantly below the ink jetting temperature. This requires that the reservoir and print head be thermally isolated and have separate heaters and temperature sensors.
Copending U.S. patent application Ser. No. 07/965,812 filed Oct. 23, 1992, for a METHOD AND APPARATUS FOR PROVIDING PHASE CHANGE INK TO AN INK JET PRINTER, which is assigned to the assignee of this application, describes an ink-jet head assembly having a premelt chamber, ink reservoir, and thermally isolated ink-jet print head. A printer using the ink-jet head assembly has start-up, idle, ready, and shutdown modes with each mode defining predetermined temperatures for the reservoir and print head. For example, in idle mode, the print head is kept at the same temperature as that of the reservoir, but when required to print, the print head temperature is rapidly elevated to bring the ink therein to its jetting temperature. The print head and its heater, temperature sensor, and temperature controller have a rapid thermal response time that reduces the time required to enter the ready mode and which acts to preserve the ink.
Phase-change ink-jet printers with reciprocating print heads produce high-quality images, but require a relatively long time to print each image. Print time can be shortened by increasing the number of jets printing the image. An ideal print head would span the full width of a print medium with ink-jet orifices spaced one picture element (hereafter "pixel") apart and would require only one scan of the print head relative to the print medium to print an image. What is needed, therefore, is a substantially media-width, multiple-orifice, ink-jet print head having a heating system that heats the print head, and the phase-change ink contained therein, to a uniform temperature throughout the print head.